Identification of hypertrophic cardiomyopathy (HCM) patients who require aggressive antiarrhythmic therapy (e.g., implantation of defibrillator device) requires knowledge of the prognosis of various subgroups of this heterogenous disease. We compared the long-term outcome in 41 consecutive HCM survivors of sudden cardiac death (SCD) with 66 HCM patients without ventricular tachycardia (VT) and 17 patients with VT on 24-hour Holter monitoring. Of the SCD patients, 23 had surgery for left ventricular outflow obstruction (OBST) and 11 patients had electrophysiologic studies (EPS). Five-year event-free (further SCD or death) rates for patients with no VT, with VT, SCD (surgical) and SCD (nonsurgical) were 90%, 76%, 67% and 37% respectively. Of the SCD who had EPS, 8 (73%) had atrioventricular conduction abnormalities and 9 (82%) patients had inducible sustained malignant VT. Thus, (a) SCD conveys a relatively poor prognosis especially in patients who were not surgical candidates, and (b) most SCD patients with HCM have both conduction abnormalities and inducible VT.